Fire alarm system



Oct 15, 1940- A. c. DONOVAN. JR 23 FIRE ALARM SYSTEM Filed July 1'7, 1937 Patented Oct. 15, 1940 UNITED. STATES PATENT OFFICE FIREALARM SYSTEM Application July 17, 1937, Serial No. 154,232

Claims.

The present invention relates to automatic fire alarm systems and more particularly to systems operating to give an alarm or other indication dependent on the rate of rise of temperature 5 within the space to be protected.

Indicators operable upon rate of temperature rise usually depend upon differential conditions arising from variations of electrical resistance in two or more conductors. The object of the present invention is to provide a simple and reliable system which is not dependent upon such differential effects, and to this end the invention contemplates as its principal feature the provision of a single conductor having a temperature ooefficient of resistance, together with means for actuating an alarm in accordance with the rate of change of resistance thereof.

Other features of the invention consists of certain novel features of construction, combinations and arrangement of parts hereinafter described and particularly defined in the claims.

The accompanying drawing is a diagram of the preferred form of alarm system according to the present invention.

The system shown in the drawing comprises an electrical conductor 4 in the form of a wire which may be strung along the wall or ceiling of a room and subject to changes of electrical resistance upon changes of temperature. The wire may be of copper or any other material having a moderately high temperature coefiicient of resistance. The conductor 4 forms one arm of a Wheatstone bridge, the other three arms being indicated at 8, 8 and Ill. The bridge is energized by a battery 35 I2 connected across the arms Band 8. An indicating circuit shown generally at I4 is connected at one side to the other two corners of the bridge. As is customary in bridge arrangements, the four arms may have such resistances that at some normal temperature the bridge is in balance, that is, there is no potential difference existing between the ends of the indicating circuit l4, although for reasons which will presently appear, the condition of balance at any temperature is not essential.

The indicating circuit includes a condenser l6 and two resistors l8 and all in series. The resistors l8 and 20 are preferably equal in magnitude. It will be seen that under constant conditions no current flows in the indicating circuit whether or not the bridge is in balance, but that any variation in the resistance of the conductor 4 causes a transient current to flow in the indicating circuit, the magnitude and duration of the 55 current being dependent upon the rate of change of the resistance of the conductor-and on therelative magnitudes of the condenser l6 and the resistors I8 and 20. Any such transient current produces a momentary potential difference across the resistors. To utilize the transient potential difference across the resistors occasioned by a change in resistance of the conductor 4, the resistors l8 and 20 are connected into an amplifying circuit which, as shown in the figure, comprises two 10 amplifying tubes 22 and 24. The junction of the two resistors is connected to the filaments of the tubes through a battery 26 arranged to impress the negative bias potential on the grid, and the outside ends of the resistors are connected to the 15 grids of the tubes as indicated at 28 and 29. The plate circuits of the tubes comprise the plate battery 3|], the positive end of which is connected through plate resistors 32 and 34 respectively to the plates of tubes 22 and 24. An alarm cir- 20 cuit 36 is connected across the ends of the resistors 32 and 34 and includes a relay 38 having normally open contacts 40 which upon energi zation of the relay may be closed to give any suitable alarm or indication. An alarm device is 5 illustrated in the drawing as a bell 42. The contacts 40 are preferably provided with a suitable latch indicated diagrammatically at 44, for holding the contacts closed following energization of the relay.

Under constant temperature conditions no current flows in the indicating circuit I4. If the bridge is in exact balance there is no potential difference across the indicating circuit and the charge on the condenser is zero. This condition of exact balance is not essential and a steady potential difference may exist across the indicating circuit in which case the condenser will have a continuous charge although, so long as the temperature conditions are constant, no current will 40 flow in the indicating circuit. Any change in the resistance of the conductor 4 results in a change of potential across the indicating circuit and this causes flow of a charging current to the condenser and through theresistances l8 and 20. 5 It will be understood that the term charging current means the transient current in the indicating circuit following any change of potential across the indicating circuit, and this charging current may be in either direction. The potential differences across the resistors l8 and 20 caused by the transient current flow result in the application of equal and opposite signal potentials on the grids of the tubes 22 and 24. Under normal conditions there is no potential difference across the alarm circuit 36 and hence no current in the relay 38, but upon the unbalance: produced by the application of opposite signal potentials to the tube grids, current flows in the alarm circuit 36. The relay 38 is adjusted as to sensitiveness to respond only to currents of a certain magnitude which depend in turn upon the rate of change of resistance of the conductor 4, and hence upon the rate of change of temperature in the space to be protected. It has been found that most satisfactory results are obtained if the system is set to respond to a rate of temperature rise of about 25 F. per minute in the space to be protected, but the invention is not limited to any'particular setting. If a rise of temperature occurs at a rate at least as great as the predetermined value, the relay is energized and the contacts 40 are closed, thereby giving a suitable alarm.

It will be understood that the direction of current flow in the alarm circuit 36 depends on whether the resistance of the conductor 4 is increasing or decreasing. An increasing resistance is indicative of an increase of temperature, and if the rate of increase is sufiicient it indicates a probable fire. A decrease of resistance is, however, not important since it may result only from the entrance of some cold air into the room. It is usually desirable to make the system responsive only to temperature increases and to this end the relay 38 is preferably polarized to respond to alarm circuit currents in only one direction.

Having thus described the invention, I claim:

1. An automatic fire alarm system comprising a conductor having a temperature coemcient of resistance, a bridge in which said conductor constitutes one arm, an indicating circuit for the bridge including a condenser and a resistor in series, whereby a change in the resistance of the f conductor causes a transient charging current to flow through the condenser and resistor, means for amplifying the potential difierence across the resistor due to the charging current, and signal means associated with the amplifier output.

2. An automatic fire alarm system comprising a conductor having a temperature coeflicient 0! resistance, a bridge in which said conductor constitutes one arm, a direct current source of potential for the bridge, an indicating circuit for the bridge including a condenser and a resistor in series, an amplifier associated with the resistor, and signal means including a polarized relay operated by the amplifier output.

'3. An automatic fire alarm system comprising a conductor arranged over the area to be protected and having a temperature coeflicient of resistance, a first circuit means constantly passing current through the conductor, a second circuit means including a condenser in series with a device responsive to predetermined values of current, the second circuit being connected to the first circuit at points such that the potential at the terminals of the condenser is dependent at least in part upon the resistance of the conductor, whereby the responsive device is actuated only by the transient currents flowing during change in the resistance of the conductor.

4. An automatic fire alarm system comprising a conductor arranged over the area to be protected and having a temperature coeflicient of resistance, a first circuit means constantly passing current through the conductor, a second circuit means including a condenser in series with a resistor, the second circuit being connected to the first circuit at points such that the potential at the terminals of the condenser is dependent at least in. part upon the resistance of the conductor, and signal means actuated by a potential difierence of predetermined magnitude across the resistor due to transient currents flowing during change in the resistance of the conductor.

5. An automatic fire alarm system comprising a conductor having a temperature coeificient of resistance, a bridge in which said conductor constitutes one arm, an indicating circuit for the bridge including a condenser and a resistor in series, whereby a change in the resistance of the conductor causes a transient charging current to fiow through the condenser and resistor, and signal means actuated by a potential difierence of predetermined magnitude across the resistor.

ARTHUR c. DONOVAN, JR. 

